research article diffractive optical elements with a large angle … · 2019. 7. 31. ·...

$: Research Article Diffractive Optical Elements with a Large Angle … · 2019. 7. 31. · holographic element, as shown below. For this study, three o-axis focussing elements are then$
Research ArticleDiffractive Optical Elements with a Large Angle ofOperation Recorded in Acrylamide Based Photopolymeron Flexible Substrates

Hoda Akbari,1 Izabela Naydenova,1 Lina Persechini,2 Sean M. Garner,3

Pat Cimo,3 and Suzanne Martin1

1Centre for Industrial and Engineering Optics, School of Physics, Dublin Institute of Technology, Kevin Street, Dublin 8, Ireland2Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, UK3Corning Incorporated, One Riverfront Plaza, Corning, NY 1431, USA

Correspondence should be addressed to Suzanne Martin; [email protected]

Received 26 August 2014; Revised 4 November 2014; Accepted 4 November 2014; Published 11 December 2014

Academic Editor: Sergi Gallego

Copyright © 2014 Hoda Akbari et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

A holographic device characterised by a large angular range of operation is under development. The aim of this study is to increasethe angular working range of the diffractive lens by stacking three layers of high efficiency optical elements on top of each other sothat light is collected (and focussed) from a broader range of angles.The angular range of each individual lens element is important,andwork has already been done in an acrylamide-based photosensitive polymer to broaden the angular range of individual elementsusing holographic recording at a low spatial frequency.This paper reports new results on the angular selectivity of stacked diffractivelenses. A working range of 12∘ is achieved. The diffractive focussing elements were recorded holographically with a central spatialfrequency of 300 l/mm using exposure energy of 60mJ/cm2 at a range of recording angles. At this spatial frequency with layersof thickness 50 ± 5 𝜇m, a diffraction efficiency of 80% and 50% was achieved in the single lens element and combined device,respectively. The optical recording process and the properties of the multilayer structure are described and discussed. Holographicrecording of a single lens element is also successfully demonstrated on a flexible glass substrate (Corning(R) Willow(R) Glass) forthe first time.

1. Introduction

Photopolymers are fast becoming one of the most popularrecording media for holographic applications for a varietyof reasons; they have excellent holographic characteristics,such as high refractive index modulation, large dynamicrange, good light sensitivity, real time image development,high optical quality, and low cost. Photopolymer materialshave been investigated for use in a number of holographicapplications [1–8]. Recent work proposes stacking of lowspatial frequency transmission holograms [9] and multi-plexing of high spatial frequency transmission Holograms[10] in different photopolymers as promising approaches fortracking-free sunlight redirecting devices.

An ideal holographic recording material should have thefollowing properties: high sensitivity to available commercial

wavelengths, high spatial resolution for improved qualityrecording, linear response to recording intensities, and lownoise, that is, a fine grain structure to reduce scatter effects.In applications where low angular selectivity is necessary, theability to record at lower spatial frequencies is also benefi-cial. Considerations regarding material flexibility, substrateflexibility, substrate thickness, substrate optical properties,and good adhesion are also of significance, particularly whenstacking layers.

Over the years a number of other researchers havestudied the response of the low spatial frequency gratings inphotopolymer materials to be used in different applications.For example, Tarjanyi et al. [11] reported on the response ofphotopolymer material at low spatial frequency and differentrecording intensities and Pascual et al. [12] have investigatedthe capability of a photopolymer recording material for mass

Hindawi Publishing CorporationInternational Journal of Polymer ScienceVolume 2014, Article ID 918285, 7 pageshttp://dx.doi.org/10.1155/2014/918285

2 International Journal of Polymer Science

production of computer generated gratings at low spatialfrequencies.

In the work presented here a self-developing acrylamide-based photopolymer has been used as the photosensitivemedium [13–18]. The capability of this material to record atlow spatial frequencies has been attributed to the relativelyhigh permeability of the polymer matrix and fast monomerdiffusion, which allows for the recording of high diffrac-tion efficiency gratings even in the low spatial frequencyregime [19]. Such low spatial frequencies are preferable whendevices with larger acceptance angle (broad angular andwavelength selectivity operation ranges) are sought. Keepingthe spatial frequency low ensures that the angular selectivityof each individual grating/lens is low so that the range ofangles accepted by each individual grating is maximized andthe number of gratings needed in the combined device isminimized. In this work we present low spatial frequencyphotopolymer holographic lenses stacked together in orderto increase the angular range of the focussing device.

2. Theoretical Background

For a given material, diffraction efficiency depends directlyon the thickness of the holograms, but the angular andwavelength selectivity depends on both thickness and thespatial frequency of the grating. In agreement with volumeholographic grating theory [20], a previous study demon-strated that gratings and lenses recorded on thinner layers atlow spatial frequencies have a much greater acceptance angle[9]. For example, the acceptance angle for gratings recordedin 50 𝜇m thick layers at a spatial frequency of 1000 l/mmis about 1∘, whereas gratings recorded at spatial frequencyof 300 l/mm show an acceptance angle of approximately 3∘.Multiplexing many gratings in one layer has been shownto work well [4]; however the diffraction efficiency of eachgrating decreases with the number of gratings recorded dueto the limited dynamic range of the photopolymer.

In order to increase the angular range further withoutreducing the diffraction efficiency of each element, a numberof holograms can be stacked by laminating them togetherusing flexible substrates. The use of flexible substrates (suchas plastic and flexible glass [21]) has significant advantagesover conventional thicker glass as is flexible, conforms to therequired shape, and has a reduced weight as well as thickness,implying lower losses for stacking devices. Examples ofstacked displays on flexible glass substrates have previouslybeen demonstrated [22]. Use of flexible substrates alsoenables high-volume continuous manufacturing methodssuch as roll-to-roll device fabrication [23].

In this study the holographic elements used are focussingelements, also referred to as diffractive optical element (DOE)lenses. DOE lenses are made by interfering a sphericallyfocussed beam with a reference beam (collimated) andarranging the photopolymer layer at the overlap as describedin Section 3.3. The photopolymer material responds by pro-ducing a local variation in refractive index that records theinterference fringe planes formed as the spherical and planewaves interfere. Controlling the interference pattern allows

Λ

𝜑

A

B

Figure 1: Schematic of the diffractive optical elements (DOEs)which redirects the incident light to an off-axis focus point, withinset showing typical fringe structure.WhereΛ is the fringe spacingand 𝜑 is the angle between the fringes and the normal to the planeof the recording medium.

control of the diffraction properties of the element recorded.The performance of the DOEs is characterised in terms ofdiffraction efficiency and angular selectivity.

In this approach each element is designed to focus anoff-axis collimated incident beam to a point behind theholographic element, as shown below. For this study, threeoff-axis focussing elements are then stacked together, eachdesigned for a different angle of incidence, so that whencombined, the stack is capable of focusing light incidentfrom a broader range of angles. This arrangement of layersalso requires a careful control of their thickness so that eachfocusing elements has the expected FWHM working range.

Figure 1 is a schematic diagram showing how the individ-ual DOE redirects the incident light. The DOE is a complexdiffractive element which can be thought of as a series ofslanted gratings that redirect the incident light towards thefocal point. The spatial frequency and slant angle of thegrating planes vary across the DOE. For example, the spatialfrequency of the grating at point A in the diagram will bemuch greater than the spatial frequency at point B, but theslant angle of the grating planes will be greater at B than A.The material used must be capable of recording the desiredrange of spatial frequencies.

In this study, in order to increase the operation angle ofthe device three individual layers of high efficiency DOEs arerecorded at three different positions of the recordingmaterialwith respect to the interference pattern. After recording theyare laminated on top of each other without any air gap inorder to reduce reflection losses.

Since they can be considered thick holograms, the max-imum diffraction efficiency is achieved when the DOEs areprobed at the correct reconstruction angle. Each gratingelement within the DOE should be illuminated at the Braggangle for that grating. This is achieved using a collimated

International Journal of Polymer Science 3

Table 1: Concentrations of the photopolymer composition.

Components AmountAcrylamide 11mmolMethylenebisacrylamide 1.3mmolPolyvinyl alcohol (10% wt/v stock) 17.5mLTriethanolamine 15mmolErythrosine B dye (0.11% wt/v) 5𝜇mol

beam in both recording and reconstruction. For an unslantedtransmission grating, the Bragg condition is given by

𝑚𝜆 = 2Λ sin 𝜃, (1)

where 𝑚 is the diffraction order (for thick holograms 𝑚 =1), 𝜃 is the Bragg angle defined as the angle that the incidentbeam makes with the fringe plane in the recording medium,𝜆 is the wavelength of the recording beam, andΛ is the fringespacing.

The recorded DOEs in this study are considered as thickholograms [20, 24–26], because 𝑄 ≥ 1, where parameter 𝑄 isdefined by

𝑄 =2𝜋𝜆𝑑

𝑛0Λ2

, (2)

where 𝑑 is the thickness of the recording medium, 𝑛0is the

average refractive index of the recording material, and Λ ispreviously defined.

In this study these parameters correspond to a 𝑄 factorof about 10. This value was estimated by taking the spatialfrequency of 300 l/mm at the centre of the lens and thephotosensitive layer thickness of 50 ± 5 𝜇m.

Kogelnik predicts the relationship between incident angleand diffraction efficiency in thick gratings, for specific con-ditions [20]. For gratings that are not overmodulated thereis a maximum at the Bragg angle and the width of thepeak depends on grating thickness and spatial frequency.The lenses presented here are characterised by measuringthe diffraction efficiency over a range of angles in orderto determine this angular selectivity. When interpreting theresults it should be borne in mind that for such DOEs a rangeof spatial frequencies and slant angles are always present.

3. Experimental

3.1. Photopolymer Solution Preparation. Thematerial used inthis research is a self-developing acrylamide-based water-soluble photopolymer as previously described [13]. The com-position of this material is acrylamide and methylenebisa-crylamide monomers, triethanolamine as an initiator, apolyvinyl alcohol binder, and an erythrosine B as a sensitizer.The average refractive index of the fabricated photopolymerlayer is approximately 𝑛

0= 1.50 [27].

Table 1 shows the component concentrations of the pho-topolymer solution used to obtain layers of 50 ± 5 𝜇m thick,as confirmed by white light interferometry.

M

S

Rotation stage

M

SF CL BS

Laser 532nm

PS

Laser 633nm

M

FL

Figure 2: Experimental set-up for recording focussing transmissionholograms: S: shutter, CL: collimating lens, BS: beam splitter, SF:spatial filter, M: mirror, FL: focusing lens, and PS: photopolymersample.

3.2. Layer Preparation. 0.5mL of the photopolymer solutionwas spread evenly using the gravity settling method on a 25 ×75mm2 flexible plastic (Bayer’s Makrofol) or glass substrateand then placed on a levelled surface and allowed to dry for18–24 hours in darkness under normal laboratory conditions(20–25∘C, 40–60% RH).

3.3. Holographic Set-Up. DOEs were recorded in a two-beam holographic optical set-up (Figure 2) using a verticallypolarized Nd:YVO

4laser (532 nm), and a Helium-Neon laser

(He-Ne) at 633 nm was used as a probe beam. Both therecording beams and the probe beam are vertically polarized.

The intensity of the recording beams was set by a variableneutral density filter to 1mW/cm2. Previous studies haveshown this intensity to be optimum for recording at aspatial frequency of 300 l/mm [9]. A spatial frequency of300 lines/mm was obtained by adjusting the interbeam angleto 9∘. The exposure time was kept constant at 60 s; thusan exposure energy of 60mJ/cm2 in a layer of thickness50 ± 5𝜇m was achieved. A rotation stage was used toset the angle of the recording medium with respect to therecording beams, for each element. In order to characterizethe diffracted intensity dependence on the incident angle ofthe probe beam, the grating was placed on a rotation stage(Newport, ESP 300). An optical power meter (Newport 1830-C) recorded the intensity of the diffracted beam and the datawas transferred to a computer via a data acquisition card. ALabVIEW program was used to control the experiment andto record the data.

The first step of this experiment involves the characteriza-tion of the Bragg selectivity curve for each layer individually.The Bragg curve can provide information about the grat-ing/lens parameters such as efficiency, thickness, angular fullwidth half max (FWHM), and refractive index modulation.The second step involves the study of the angular selectivityof the device after stacking three DOE lenses, recorded inindividual layers, on top of each other. The distance fromthe photopolymer sample to the focal point of lens defines


the focal length of the recorded DOE, where in this study thefabricated elements have a focal length of about 6 cm. Thefocal length of fabricated DOEs can be varied by changingthe distance between the focus point of the lens to thephotopolymer recording material.

The diffraction efficiency of the recorded DOE lensesdetermines the efficiency of each element at redirecting lightto the appropriate angle and is defined as

𝜂 =𝐼𝑑

𝐼in× 100, (3)

where 𝐼𝑑is diffracted beam intensity, 𝐼in is incident (probe

beam) intensity, and 𝜂 is diffraction efficiency. Using the ratioof the intensity in the diffracted beam to the incident intensitymeans that reflection, absorption, scatter, and other losses inthe substrates and photopolymer will lower the diffractionefficiency value, but it allows a more realistic estimate of theusefulness of the elements in collecting light.

A range of high efficiency DOE lenses were holograph-ically recorded at three different positions of the recordingmaterial with respect to the interference pattern createdby the recording beams. The central spatial frequency wasrelatively low −300 l/mm. The layers then were stacked ontop of each other. Lamination of the photopolymer layer tothe substrate of its immediate neighbour in the stack ensuresgood adhesion and no air gap.The key challenge is to controlthe working ranges of the individual holographic elementsso that when laminated together the Bragg selectivity curvesoverlap sufficiently. In other words, as the angle of incidencechanges and the first grating efficiency begins to drop thesecond should begin to rise.

The experiment was carried out by characterizing theangular response of the zero diffraction order and the firstdiffraction order of the recorded lenses before and afterstacking. In this work, optical elements were aligned by usingthe grating/lens edge as a guide. Angular alignment dependson the effective lamination of each layer to the next andcareful control of the recording angles for each diffractiveelement.

4. Results and Discussion

The following results show how diffraction efficiency varieswith angle of incidence for individual holographic focussingelements, recorded in photopolymer on plastic substrates,as well as a combined stack of three elements. Successfulrecording is also demonstrated in photopolymer on flexibleglass substrates using the same techniques.

4.1. Comparison of the Acceptance Angle of Diffractive OpticalElements before and after Stacking. The variation of thediffraction efficiency with angle of incidence in the zero andfirst diffraction orders for holographic lenses recorded atrange of angles (7∘, 10.5∘, and 14∘) is shown in Figure 3. Adiffraction efficiency of about 80%was observed.TheFWHMis between 4.5 and 5∘ for each lens element.

Figure 4 shows the variation of diffraction efficiency withangle of incidence for the zero and first-order diffraction of

Table 2: The comparison of transmittance and reflectance of threesubstrates.

Substrate Transmittance Reflectance Refractive indexStandard glass 90.1 ± 0.6% 6.8 ± 0.5% 1.5Plastic 89.6 ± 1.6% 9.7 ± 0.7% 1.503Flexible glass 91.9 ± 0.7% 7.2 ± 0.9% 1.504

the stacked elements from Figure 3. It can be observed thatthe FWHM was increased to collect light from a workingrange of approximately 12∘; however, losses in diffractionefficiency have occurred in the gratings after stacking asthe diffraction efficiency has dropped to approximately 50%.Even though the layers appear to laminate together excep-tionally well this reduction in efficiency may be caused bycumulative losses (scattering and reflection) at the multiplelayer interfaces, since there are six interfaces in total includingsubstrates.Thedecrease in diffraction efficiency after stackingthe layers may be improved by using a different substratewhere the refractive index will be better matched andmovingto a thinner more transparent substrate with reduced bire-fringence, haze, and scattering. Flexible glass, such as WillowGlass, could be the solution. Table 2 displays the measuredoptical properties of three substrates.

The transmittance and reflectance of the three substratesavailable have been compared by using a laser with wave-length of 633 nm incident at 10.5 degree on the substrateand the results are shown in Table 2. Each transmittance andreflectance value is an average of 8 readings. The refractiveindex of the plastic and flexible glass substrates wasmeasuredusing an Abbe refractometer. The results are given in Table 2.

4.2. Recording High Efficiency Diffractive Optical Elementson Flexible Glasses Substrate. This section describes anexploratory study demonstrating successful holographicrecording of focusing elements with low spatial frequencyand broad working range on flexible glass (Corning(R)Willow(R) Glass) [21]. This novel glass is beneficial forour purpose as it provides better transparency (Table 2) aswell as previously mentioned advantages. The flexible glasssubstrate is dimensionally stable to enable required layer-to-layer alignment, while also displaying low scattering, haze,and absorption. The flexible glass substrate has been demon-strated to be compatible with roll-to-roll and sheet-levelcoating, lamination, printing processes, as well as stackedmultilayer devices with low parallax [23, 28].

A holographic lens with an off-axis focusing effect wasrecorded on Willow Glass of size 26 × 76mm2 and 100 𝜇mthickness. The layer preparation conditions were identical tothe conditions used for the plastic substrate. The diffractivelens element was recorded at 10.5∘ away from unslantedposition in order to compare the functionality and theperformance of the lens element with that recorded on aplastic substrate. Figure 5 shows the angular response of thezero and 1st diffraction order of the recorded lens element.The focal length of the recorded lens element was 6 cmwith diameter of 0.9 cm. It can be observed that a maxi-mum diffraction efficiency of over 90% was achieved, where


100

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First diffraction order100

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(%)

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Angle (deg)

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(c)

Figure 3: Angular selectivity curves for diffractive lenses with spatial frequency centred at 300 l/mm recorded in layers with thickness of50 𝜇m on a plastic flexible substrate, at range of angles: (a) 7∘, (b) 10.5∘, and (c) 14∘ before stacking.

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Angle (deg) Angle (deg)−5 302520151050

Figure 4: Angular selectivity curves for range of combined lens elements recordedwith the exposure energy of 60mJ/cm2 at spatial frequencyof 300 l/mm.

the FWHM was approximately 5∘. These very promisingresults demonstrate that flexible glass is a suitable and stablesubstrate for holographic recording, while achieving highdiffraction efficiency with similar working ranges to the lenselements recorded on plastic substrates.

These encouraging results will prompt new avenues ofresearch into the development of DOEs. The high opticalquality of flexible glass (low scattering, birefringence, and

absorption) will enable the fabrication of improved devicesin any application where device thickness/weight is an issueand where optical losses should be minimized. In addition,the conformable nature of holographic photopolymer devicesmade on optical glass will enable new device configurationsincluding laminated stacked holographic devices, deformableholographic devices, and combinations of holographic andoptoelectronic devices.


100

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(%) Zero diffraction order

Angle (deg) Angle (deg)

Figure 5: Angular selectivity curves for lens elements recorded at an angle of 10.5∘ on flexible glass substrate with the exposure energy of60mJ/cm2 at spatial frequency of 300 l/mm.

Future work will involve recording a series of grat-ings/lenses on flexible glass and comparison of the loss indiffraction efficiency after stacking the layers with the resultsabove. Investigation into the exploitation of the flexible natureof the glass in order to tune the holographic properties is alsocurrently underway.

5. Conclusion

Results confirmed that stacking three holographic elementson plastic substrates increased the acceptance angle of thecombined device to 12∘ FWHM. The proposed methodcould be used in applications such as a solar collectionand manipulation of beams in illumination systems. Forthe first time DOEs have been recorded on these flexibleglass substrates (Corning(R) Willow(R) Glass). A maximumdiffraction efficiency of 90% was observed in photopolymerlayers of 50 ± 5 𝜇m thickness at this spatial frequency. Thismeans that over 90%of the incident light wasmeasured in thediffracted beamwith no correction for reflection, absorption,or other losses.The focal length of the fabricated elementswas6 cm.

Future work will focus on improvement of diffractionefficiency after stacking the layers with large angular responseusing flexible glass.

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper.

Acknowledgments

Theauthorswould like to acknowledge theDublin Institute ofTechnology, Fiosraigh 2013, for financial support and also theFOCAS Research Institute for providing research facilities.The authors wish to thank Mr. Christopher Smith, TrinityCollege Dublin, for his advice and support in setting upthe collaboration with Corning. Some of these results werepresented in Cost 1205 meeting in April 2014.

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research article diffractive optical elements with a large angle … · 2019. 7. 31. ·...

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